AD421
Battery Backup
Figure 19 shows an application circuit for the AD421 where the
micro and memory sections of the circuitry are protected against
losing data if the loop is broken. The backup circuit switches
from VCC to battery voltage without a glitch when VCC power is
lost. The IRFF9113 acts as a current source during normal
operation and provides a constant charging current to the supercap
or Nicad. The loss of VCC drops the IRFF9113’s gate voltage to
zero volts, which allows the battery or supercaps current to flow
through the MOSFETs channel and integral body diode to
provide power for the micro and memory sections. To calibrate
this circuit, connect an ammeter in series with the battery or
supercap. Then with VCC and the load present adjust the 100 kΩ
potentiometer for the battery charging current recommended by
the battery or supercap manufacturer.
DN25D
IN3611
IRFF9113
0.1F
4.7F
IN3611
V
LOOP
V
CC
100k⍀
2.2F
MICRO/
MEMORY
V
CC
SUPERCAP
GND
DRIVE
AD421*
LOOP
RTN
COM
*ADDITIONAL CIRCUITRY OMITTED FOR CLARITY
Figure 19. Battery Backup Circuit
Nonrechargeable batteries should not be used in this application
due to danger of explosion.
–13–
REV. C
ADI [ ADI ]
